In industrial plants, coal, industrial waste, domestic waste, oil, peat, biomass or any other materials are utilized for combustion in boilers in order to operate the plants and produce electricity or other products. However, combustion of such materials produces flue gas that contains dust particles, which ultimately emit into ambient air adding pollution to the surrounding environment. In order to keep such emission at low level, the industrial plants most widely utilize Electrostatic Precipitators (hereinafter referred to as “ESP”). The ESP is a device that collects the flue gas from the boiler and removes the dust particles from the flue gas using the force of an induced electrostatic charge.
The ESPs are generally designed for a particular coal type or a coal range and process conditions as the dust collection efficiency of an ESP greatly depends on characteristic of the dust it handles. However, during these days the industrial plants or utility industries frequently change the coal type due to economical pressure. Mostly the shift is toward cheaper coals that produce high resistive dust which are often difficult to collect by the ESP. In view of the said problem, the conventional ESPs need to be upgraded from time to time to make it more efficient towards the collection of the dust. However, such up gradation often means expansion of ESP which is quite expensive.
Further a method that is widely used to improve performance of the ESPs is to condition the flue gas using Flue Gas Conditioning Agents before it enters into the ESP. Some of the most common used Flue Gas Conditioning Agents (hereinafter referred to as “FGCA”), are SO3, NH3, and water. The conditioning is generally carried out by injecting any of the FGCAs or combinations thereof to the flue gases before it enters the ESPs. The FGCA improve the ESP performance by changing dust characteristics favorably.
In conventional methods, for conditioning the flue gas, the injection quantity of FGCAs is generally decided based on parameters, such as a power consumption of transformer of the ESP, opacity at stake of the industrial plant, or combination thereof
However, these methods may not be the most optimal because these methods use the parameters whose values depend upon several factors besides dust characteristics. For example: when the parameter power consumption of transformer is in use, generally the FGCAs injection quantity is kept inversely changing with the power consumption. This may lead to wrong injection quantity as the power consumption variation may also be due to some other reasons, such as, high spark rate, poor gas distribution etc. beside change in dust or flue gases property. Further, most commonly used opacity signal may also not give a right indication of injection need. Sometime, a wrong opacity signal resulting in a high FGCA injection may be given due to NOx fumes in the stakes.
Boiler load is independent of dust characteristics and use of only boiler load for deciding injection quantity may go very wrong with change in process conditions such as fuel. All these parameters depends on one or more variables beside dust characteristics, therefore chances of injecting the accurate FGCAs based on these parameters may be not be good as other variables are acting constantly. To be on safer side, often, operators of the industrial plants choose to over inject the FGCAs rather than risking penalties due to high emissions. This results in high operating cost due to excess consumption of FGCAs as well as significantly increased power consumption in the ESP.
Beside this, the injection with conventional controllers may need continuous intervention of the operators to optimize, if a major process change occurs. In Industrial plants, these process changes occur very frequently, which requires continuous attention of the operators on an FGCA system to manually adjust the settings to keep FGCAs injection and emission at acceptable level.
It is important that the FGCAs are injected or added in the dust laden flue gas stream in optimum quantity as the quantity lower than necessary may not result in optimum performance of the ESP, and the quantity more than necessary may result in high cost of agent, high power consumption in the ESP, corrosion in the ESP, and dust built up etc. The existing methods of controlling the injection rate of FGCA by linking it to parameters such as transformer power consumption or opacity or boiler load or combination thereof is not enough to obviate the problems in the conventional industrial plants.